1. Field of the Invention
The present invention relates to a method of manufacturing a master plate, a method of manufacturing a microneedle patch used as a tool for administering drugs to a specified layer in the skin, a microneedle patch and an exposure apparatus.
2. Description of the Related Art
A liquid drug has been usually applied on the skin for administering the drug through the surface of a living body such as the skin. Application of the drug to the skin is a non-invasive method, and is excellent in that the method gives no stress on a patient. However, the applied drug is easily removed by sweat or touch. Daily repeated application of the drug involves problems in terms of convenience and safety. It is another problem that control of permeability is not easy when the drug is to be permeated into the inner layer of the skin.
Under the circumstances, a microneedle having a transport function of the drug has been proposed. The microneedle may be used for transport of various substances such as sampling of the blood. Although a method of permitting the drug to infiltrate into the skin using microneedles or a microneedle patch on which the drug has been applied in advance is not a perfectly non-invasive application method of the drug, stimulation to the patient is relatively small since the drug is injected into a shallow region of the skin such as the epidermis using an ultra-fine microneedle. The drug may be infiltrated with high efficiency as compared with the method of applying the drug merely on the surface of the skin.
There is known a method of manufacturing a microneedle in which silicon is subjected to dry etching (Devin V. McAllister et al., PNAS, Nov. 25, 2003, vol. 100, No. 24, 13755-13760; Shyh-Chyi Kuo et al., Tamkang Journal of Science and Engineering, Vol. 7, No. 2, 95-98 (2004)).
While a quite fine and complicated planar pattern may be formed on the surface of a photoresist film or substrate by these methods, the three-dimensional shape obtained is columnar, and the depth of a recess to the surface is fixed to be constant. This is because, while the planar shape may be freely changed in accordance with a photomask, there was no method of freely forming the recess in a depth direction. Consequently, a cone shape such as a round cone and square cone cannot be formed, and it is difficult to apply the method of the manufacturing the microneedle. Since etching residues are left behind on the surface when the surface of the silicon microneedle is observed with an electron microscope, the surface is roughened due to the residues. Consequently, high surface roughness remains on a replication plate and on the microneedle manufactured using the silicon microneedle as a master plate.
It has been known to use a method referred to as LIGA (Lithographic, Galvanoformung and Abformung) to manufacture a microneedle (Moon, Sang-Jun et al., Transducers '03, 3E95.P (The 12th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003; Jpn. Pat. Appln. KOKAI Publication No. 2005-246595).
The method of manufacturing the microneedle by LIGA involves the following steps. An X-ray sensitive photoresist such as a polymethyl methacrylic (PMMA) resin is applied on the substrate. Island radiation shields made of a material such as gold are applied on the resist, which is irradiated with X-ray having a high parallel nature. A three-dimensional structure made of PMMA is obtained upon development. A master plate is manufactured by electroforming of nickel on the three-dimensional structure. It is an advantage of this method that the tip of the microneedle may be sharpened with a smooth surface.
However, synchrotron radiation equipment that is large size equipment is necessary for the LIGA process. Accordingly, this method is inappropriate for manufacturing the microneedle patch (microneedle array) that is required to be manufactured with a low cost on a large scale.
The conventional methods have complicated steps of manufacturing the microneedle patch, and require large equipment to manufacture the microneedle patch having small surface roughness.
It is promising to manufacture the master plate for microneedle patch through exposure to a resist film as described above. Jpn. Pat. Appln. KOKAI Publication No. 2001-356187 discloses a XYθ stage apparatus used for the exposure apparatus, which precisely moves an object to be processed in a desired X-axis position, Y-axis position and θ-axis position at high speed. The XYθ stage apparatus includes a pair of linear motors disposed in an opposed relation to one another in the X-axis direction, a pair of linear motors disposed in an opposed relation to one another in the Y-axis direction perpendicular to the X-axis, a movable table on which the object to be processed is mounted, rotating support means for rotatably supporting the movable table in the θ-axis direction in the same plane as the X- and Y-axes, a XY encoder, a θ encoder, and a positional controller for controlling the X-axis position, Y-axis position and θ-axis position of the movable table by independently actuating the two pairs of liner motors.
However, it was difficult for the conventional exposure apparatus to expose the photoresist into a desired shape such as a cone. It was particularly difficult to manufacture a structure in which the same shapes are arrayed in parallel such as the microneedle array.